Electrical measuring instrument.



- No. 817,857. PATENTED APR. 17, 1906.

J. M. LEA.

ELECTRICAL MEASURING INSTRUMENT. APPLICATION FILED JULY 31, 1905.

3 SHEETS-SHEET l.

'PATENTED APR. 17

J. M. LEA. ELECTRICAL MEASURING INSTRUMENT.

APIfLIOATION FILED JULY 31, 1905.

3 SHEETS-SHEET 2.

.No. 817,857. I PATENTED'APR.-17,190'6.

: J. M. LEA. ELECTRICAL MEASURING INSTRUMENT.

APPLICATION FILED JULY 31, 1905.

3 SHEETS-SHEET a J rrnn srm nsgr a rnnr oriri o JOHN M. LEA, OF CHICAGO,ILLINOIS, ASSIGNOR TO INTERNATIONAL ELECTRIC METER COMPANY, OF CHICAGO,ILLINOIS, A CORPORA- TION OF ILLINOIS.-

ELETRICAL a'ieasume unerauaianr.

Specification of LettersPatent.

Patented April 17, 1906.

Application filed July 81, 1905- Berial No. 271.922. 7

To all whom it may concern.-

Be it known that 1, JOHN M. LEA, a citizen of the United States,residing at Chicago, in the county of Cook and State of Illinois, haveinvented a certain new and useful improvement in Electrical MeasuringInstruments, of which the followingis a full, clear, concise, and exactdescription.

My invention relates to an electrical measuring instrument of the classin which a coil adapted to carry the current to be measured, or adefinite portion of such current, is arranged to turn in the field offorce of a magnet; and the invention relates more particularly to aninstrument of the so-called gravity type in which the movement of thecoil in the magnetic field is opposed not by springs, but by a weightacting on the moving system tending to maintain the coil normally at thezero position.

One feature of the invention involves an improved construction of themagnetic system whereby the deflections of the coil may be madesubstantially proportional to the strength of the current passingthrough .the same without sacrificing proper the moving system.

Another feature of the invention contemplates an improvement in thedisposition or arrangement of the weighted arms, so that when the coilis in the zero osition the Weight may be given difierent ad iustments tovary the resistance which it will offer to the movement of the coil,such adjustment being made without shifting the center of gravity of themoving system in such a way as to change the normal zero position of thecoil to a harmful degree.

A third feature of the invention relates to the construction by whichthe ends of the.

Figure 1 is a front view of the instrument with the cover removed. Fig.2 is a erspective view indicating the manner in w 'ch the moving systemmay be inserted in and removed from its place between the pole-pieceslightness ofpieces 9 9 of a permanent magnet 10.

or" the magnet. Fig. 3 is a detail perspective view of one end of thecoil-frame much enlarged to illustrate the disposition of the ends ofthe coil. Fig. 4 is a detail end view of the coil-frame alsoconsiderably enlarged. Fig. 5 is a sectional elevation of the movingsystem and its associated parts. Fig. 6 is a diagram illustrating theconstruction whereby the Weight whichopposes the movement of the coilmay be adjusted without varying the zero position of said coil to adetrimental degree. Fig. 7 is a diagram illustrating the construction ofthe pole-pieces, which makes it possible for the movements-of the coilto be made substantially proportional to the current passing through it,which causes the defiection. Fig. 8 is a detail view illustrating themanner of attaching the needle and hub to the coil-name.

The same characters of reference are used to designate the same partswherever they are shown,

The moving coil is wound upon the inner side of a coil-frame 8 ofnon-magnetic metal, which is pivoted upon its axis to turn in themagnetic field between two opposing pole- The coil-frame, as shown, isrectangular and embraces'a cylindrical core 11, which being locatedbetween the pole-pieces 9 cooperates with them to form a substantiallyannular held of force in which the coil-frame turns ivotal.

which the coil-frame'turns. Theyoke-piece 13, together with the core andthe movingcoil, constitute the movement,' which is adapted to besupported between the polepieces 9 by screws 13 passing through saidyoke-piece into the polepieces 9. l By reference to Fig. 2 particularlyit will be seen that the movementmay be readily inserted in or removedfrom the magnetic system by sliding longitudinally between thepole-pieces.

As shown. most clearly in Figs. 3 and 4, the ends of the wire formingthe coil are led out around opposite edges at the-endof the coil frameand are secured on either side of the axis of the coil-between insulatedwashers 14.

By this construction the coil-frame is kept sectional view Fig. 5, theinsulating-washers 14, between which the ends of the coil are secured,may be held between the shoulder of the hub 15 and the coil-frame 8, towhich said hub is riveted. The current may be led to and from the coilby means of very fine flexible conductors 19, which are soldered to theprojecting ends of the; coil held between washers 1 1-, said flexibleconductors 19 being anchored upon an insulating-piece 20, sup- I portedupon one of the arms 12 of the U- shaped bracket 13. The flexibleconductors 19 simply serve to convey the current and do not resist orassist to an appreciable degree the movement of the coil in itsbearings.

One of the features of this invention consists in the formation of themagnetic circuit in such a way as to establish a field graduallydecreasing in strength in the direction in which the coil tends to turnunder the influ ence of the current passing through the same. This maybe accomplished by cutting away the inner faces of the pole-pieces 9 insuch a manner that the air-gap between the faces of said pole-pieces andthe core 11 'will be considerably wider toward the end of the path inwhich the coil moves under the current v than at the beginning of saidpath. The reason for this construction is as follows: I have found thatin the gravity type of instrument if the magnetic field through whichthe coil moves is of uniform density (as commonly provided) thedeflections of the coil for a given increase of current will be muchgreater toward the end of the arc of movement of the coil than they areat the beginning. In other words, the scale of the. instrument wouldhave much wider divisions toward the end. The reason for this is thatwhen a comparatively light opposing weight is used the resisting momentdeveloped thereby as the coil is turned does not increase proportionallywith the tor ue developed by the passing of the current. fhua'referrinto Fig. 7, normally the center of gravity of the Weighted coil, which isfree to turn, will lie below the axis of said coil in a vertical planepassing throu h said axis for instance, at the point- 0,. This will bethe zero position of the coil. Now as the coilrota'tes upon its axis inthe direction of the arrow the center of gravity is moved from the pointa in a quadrant or are toward the point I). As the coil is turnedthroughthis quadrant it will be readily understood that, the resistance-causedby avity opposing therotation of the coil wi l increase not directly inproportion to the lengthited by the dimensions of the containing-case ofcourse a sufficient resisting moment might be obtained; but in the typeof instrument under consideration, where the moving systom must be keptextremely light, so as to require very little expenditure of ener y whenoperatin it, the use of a heavy weig t is objectionable, so that ingravity instrumentsf of this type heretofore in use it has been commonto calibrate the scale with unequal divisions increasing in lengthtoward the end of the scale.

In accordance with my invention I so form the ma netic system that thefield through which t e coil moves will radually decrease in strengthtoward the emf of the arc of such movement, so that a given increase ofcurrent will produce a torque substantially proportional to theresisting moment developed 5 y the very light Weight employed. By thismeans I am enabled to secure a deflection of the coil through an arcwhich will be sub stantially proportional .to the increase of currentthrou h the coil, no matter in what part of the fie d the coil may lie,whether at the beginning or near the end of its arc of movement. Asshown in the drawings, this decrease in the stren th of the field issecured by increasin the wi th of the air-gap toward the end of t e areof movement of the coil, this'increase in the width of the airap causingan unequal distribution of the ux, such that the density. decreasestoward the end of the are. This increase in the width ofthe airgap maybe provided by cutting the curved. inner face of the pole-pieceseccentric tothe arc in which the coil moves.

The degree -of eccentricity can readily be determined by experiment forany given size of instrument. sirable to cut away the pole-pieceeccentrically only alon the latter half of the quadrant through whichthe coil turns, as during the first forty-five degrees of the arc theresisting moment is substantially proportional to the torque produced bya given increase of current when suitable weight is used. In practice Iform the pole-pieces by first millin the faces thereof concentric withthe axis 9. out which the coil is to turn, after which I use a slightlylarger milling-cutter set over a. sufficient distance tocut away theportion of the pole-face toward the end of the are through which thecoil is to swing without changing I have found that it is dei the weight23 may the curvature of the first half of the quadrant. Thus withreference to Fig. 7 the first cut of the pole-piece will be made by acutter set on the center a, after which the pole-faces are finished byslightly larger cutters set on the centers 0 and 0 respectively, eachpole-piece I of course being milled separately.

The system of weights for the coil is indicated in Fig. l and in thediagram Fig. 6. The weight 21 is a counterbalance for the needle 22 andfor a second Wei ht 23, which is adjustably mounted upon theverticallydepending frame 24. It has been the practice heretofore toprovide two weights for the moving element of the measuring instrument,said weights bein mounted on arms set at a right angle to eac other, sothat by adjusting the weights along said arms the center of gravity maybe shifted as desired.

- My invention involves the further improvement that the weight whichserves to oppose the motion of the coil may be adjusted toward or fromthe axis of the coil to vary the resisting moment which it will offerwithout shifting the center of gravity of the moving system from avertical plane, and so without changing the zero position of the coil.For this reason the'arm 2 1, which carries the weight 23, is bent sothat the outer ortion thereof hangs normally in a substantia ly verticalposition.

By reference to Fig. 6 it will be seen that be moved a considerabledistance along the vertical portion of arm 2% while the coil is in itszero position without 1 changing the effect of this weight as a balanceto the weight 21, and so without changingthe zero position. Thisvertical adjustment of the weight 23 will, however, vary itsradialdistance from the axis of the coil, and so will vary the leverage whichit has upon the coil tending to oppose the rotation thereof. It willthus be seen that after th. weight 21 is adjusted to properlycounterbalance the needie 22 the weight 23 may then be adjusted to exertany desired opposing force to the rota-v tion of the coil withoutchanging the zero position thereof. This will be esirable, for example,in case of a change of strength of the relative forces acting after theinstrument has been calibrated, as by this adjustment the readings ofthe instrument may be brought again to respond to the ori inalcalibration. Such an adjustment is a so desirable duringassemblyof theinstrument to compensate for variations which may occur in thedimensions or relative positions of the parts during the process ofmanufacture.

The indicating-needle 22 is secured to the hub 16 preferably as shown in.Fig. 5that is, by being passed through a transverse hole in said hub.The lower end of the needle projecting beyond the hub is threaded, asshown, to receive the counterbalance-weight 21. In cutting thethreads'on the needle the mate- .secured to the moving s coil-framehaving 1 rial is slightly burred, so that the effective diameter of theneedle near the upper end is slightly larger than the unthreadedportion. In manufacture a hole may be bored through the hub 16 of justthe right size to receive the needle, which is drawn through the hole(the spade end 25 of the needle not being yet attached) and is pulled,through the hub until the threads formed at the lower end ofthe needleare jammed into the hole, thus making a tight fit. After the needle isin position it may further be secured, if desirable, by a drop ofshellac or the like.

The weight-arm 24 may conveniently be V stem by being fastened betweenthe shou der of the hub and the 'surface of the coilframe. The rivetingof the hub to the coil-frame serves to bind the end of the weight-armfixedly to the coilframe. The weight arm 24 is preferably threaded itsentire length. for convenience in manufacture, and also to securerigidity by the flow of a sticking compound, such as shellac, betweenthe threads where said arm is riveted.

I claim- 1. In an electrical measuring'instrument, the combination witha coil pivoted to turn upon a substantially horizontal axis, of a weightcarried by said coil, acting to maintain said coil normally in a zeroposition and opp'osin the rotation thereof, a permanent magnet avingconcave opposite polar faces, a core between said polar faces, aboutwhich said coil is axially rotatable, said polar faces being shaped toestablish a magnetic field decreasing in strength in the direction inwhich the coil turns under the influence of an electric current passingthrough the same; whereby the deflections of said coil are renderedsubstantially proportional to the current.

2. In an electrical measurin instrument, the combination with a coil anan indicator adapted to be moved thereby, of a central core about whichsaid coil is axially pivoted to turn, a weight for yieldin lymaintaining said coil at a zero position, an a magnet havin concaveopposite polar faces on o posite si es of said core, curved eccentricaly with respect to said core to produce an air-ga in-. creasing in widthtoward the limit 0 the movement of said coil therein.

3. In an electrical measuring instrument, the combination with amovingcoil, of a pivot-hubs at the ends thereof, a pair 0 nsulating-washers atone end of said hubs, the ends of the wire forming the coil beingsecured between said washers, and fine flexible conductors fastened tosaid wire ends and anchored at their other ends to a sup ort. a 4. n'anelectrical measuring instrument,

supported, hubs riveted to opposite ends of said frame, jewel-bearingscarried by said hubs, fixed pivot-screws having points entering saidjewel-bearings, a pair of insulatingwashers interposed between thecoil-frame and one of said hubs, the ends of the wire of the coil beingpassed over the edge of the coilframe and across the end thereof betweensaid washers, on opposite sides of said hub,

and fine flexible conductors fastened to the .a pivotal hub therefor, atransverse hole I throughsaid hub, and an indicating-needle having athreaded end, said needle bein drawn through said hole until saidthreade end jams therein, whereby the needle is fixed in position.

6. In an electrical measuring instrument, the combination with thecoil-frame, of a hub riveted thereto, and a weight-arm bound between theshoulder of the hub and said coilframe and fastened by the rivetingthereof.

- In witness whereof I hereunto subscribe my name this 25th day of July,A. D. 1905.

JOHN M. LEA.

Witnesses:

AUGUST BENSON, DE WIT-T C. TANNER.

